Borehole seismic investigation is among the tools that oil and gas professionals use to assist them in understanding formation properties. Seismic investigation typically involves generating acoustic waves that travel downwardly into the formation by firing a source placed at the surface of the formation. The acoustic waves are reflected back upwardly toward the surface by subterranean layers of rock. Receivers may be positioned in the borehole to record data relating to the reflected waves, which data geophysicists may use in an attempt to gain a picture of the subterranean structure of the formation.
Borehole seismic investigation has been adapted to monitor microseismic events, including natural and artificially triggered events such as when fluid is produced from or injected into a naturally fractured reservoir. Because the timing of microseismic events cannot be predicted, recording systems are typically active for extended periods of time. For example, when monitoring microseismic events during a hydraulic fracturing job, the total pumping time in which fracturing may occur can last several hours. During this time, a few tens of microseismic events, which have a duration on the order of seconds, may be recorded. Consequently, in a typical project, about 99% of the data transmitted to surface contains nothing of interest. The large volume of data generated by an array of microseismic acoustic sensors in a well requires a sophisticated telemetry system. Even so, telemetry may still be the bottleneck for real-time data acquisition. Downhole data filtering may allow transmission of only the pertinent data and may remove the data rate constraint of the telemetry system. Typical downhole filtering/data compression systems, however, are based on analysis of the acquired data in order to identify an event of interest and event identification is normally performed by an experienced analyst and can be very difficult to automate.